1. Field of the Invention
This invention relates generally to the art of reverse osmosis and ultrafiltration, and in particular, to reverse osmosis, ultrafiltration, or microfiltration devices employing an envelope of semipermeable membrane sheets spirally wound or wrapped about a tubular mandrel. The convolutions of the wound membrane sheet or sheets are separated from one another and a feed solution is introduced therethrough with permeated fluid passing through the membranes by a pressure driving force and discharged into the hollow mandrel.
2. Description of the Prior Art
In reverse osmosis, ultrafiltration and microfiltration devices, an impure solution or a solution to be concentrated is brought into contact with a semipermeable membrane. A pressure is applied to the solution to force liquid (the permeate) through the membrane, thereby filtering or concentrating the initial solution. Membranes can be selected for a particular use by comparing the average porosity of the membrane and the size or molecular weight of the solute or particles of the starting solution. Moreover, membranes may be of two different shapes, e.g. hollow tubes or flat sheets. The flat sheet membrane can be installed in devices in spirally-wound, or with plates and frames configurations. In the spiral-wound system, one or more flat sheets of membrane material are wound around a perforated permeate collection tube. Fluid flow through the modules is unidirectional, i.e. permeate passes through the membrane to the collection tube, while the concentrated residue passes along one side of the membrane to be collected or discarded. The membrane sheets are sealed on three of the four edges and the fourth edge is sealed to the collection tube and communicates therethrough through the perforations.
Examples of prior art spiral-wound modules are described in U.S. Pat. Nos. 3,695,446; 3,827,564; 3,813,334; 3,928,204; 3,367,504; 3,173,867 and 2,599,604. Examples of patents providing a meandering course for the flow of feed fluids are shown in the Newman Patent No. 4,053,418 and the French Publication No. 2,211,274 published July 19, 1974.
The prior art devices, as exemplified by the above-referenced patents, in general, utilized grid-like or mesh layers for maintaining the gap adjacent the semipermeable membranes. The mesh or open grid separator defines the feed water channel dimensions. Unfortunately, the flow of fluid across the surface of the membrane was controlled by and often hindered by such separators.
The surface of the membrane must be washed with the flow of the feed fluid across the surface of the membrane to prevent concentration polarization of ionic salts and suspended solids. In the case of reverse osmosis membranes and salt solutions a velocity of 0.5 to 1 foot per second is sufficient to minimize concentration polarization when the feed water does not contain suspended solids. When suspended solids are present, they tend to concentrate at the membrane surface as the water passes through the membrane. Even in very low concentrations the solids can form a film on the surface which reduces the production rate of the membrane. Increasing the transverse velocity of water across the surface of the membrane reduces the thickness of the film of suspended solids and maximizes the production rate. Conventional limits of suspended solids at the low velocities is a Silt Density Index of less than 5 and a maximum turbidity of one Nephelometric Turbitiy Unit (NTU), when using conventional commercial spiral wound reverse osmosis elements. This usually represents a suspended solids concentration of less than 1 part per million (ppm).
In many cases, particularly where there are relatively few suspended solids, the usual grid or mesh separator sheets that are found in commercial reverse osmosis elements will suffice to provide sufficient flow of fluids. However, it has been found in certain installations, and in particular where it is desired to pretreat water taken from flowing rivers and the like, the source is often quite turbid. Conventional devices tend to foul quickly with the mesh or grids of the separators becoming clogged, and the membrane becoming fouled, thereby reducing the operating hours between cleanings, increasing the operating pressure to maintain a given production rate, or reducing the production rate. To achieve higher velocities a open feed channel separator is required. This has normally been accomplished with plate and frame hollow tube or hollow fiber devices. However, spiral wound devices are less expensive to construct since they normally use a tube as a container.
The Newman Patent No. 4,053,418 discloses a coiled dialyzer used in artificial kidney systems wherein there is an embossed support member constructed to prevent the membrane from contacting the web of the same support member to assure uniform dialysis flow between the membrane and the web. In this case the separator is in the form of an imperforate, impermeable web and includes integrally formed embossed ribs which define angular flow channels, and also act to separate the support or web from the semipermeable membrane. The Heden U.S. Pat. No. 3,352,422 provides "saw tooth" obstruction in "plate and frame" type of dialysis apparatus. Thus a cylindrical disk includes a center hole for passing through one of the flows of fluid and spirally formed grooves, the inner end of the same being placed in the proximity of the center hole. The saw-tooth formation is applied to the edges or ridges of the grooves.
A spirally wound membrane construction was also disclosed in the French Publication No. 2,211,274, wherein the membrane construction included a tubular member comprising two tubular sections separated from one another by means of a cylindrical watertight plug inserted into one end of each section. Each section includes a lateral or side opening communicating with a recessed passage in the membrane composite. The continuous recessed passage is generally U-shaped, with a separate tongue-like portion extending laterally from the area of the plug when one end of the membrane composite has been secured to the tubular mandrel. Thus, the fluid to be separated or filtered enters one section of the mandrel, leaves that section via its lateral opening and into one side of the U-shaped passage and around the projecting "tongue" to exit on the other side and into the lateral opening of the opposite tubular section.
The aforementioned Newman and French patents are examples of open feed channels where guidance ribs or paths are used. Their methods of construction are relatively expensive to manufacture. There are other limitations to their respective designs. The Newman design is intended for dialysis applications where differential pressures are low and the membrane homogenous. In reverse osmosis applications, where the differential pressure or driving force across the membrane is on the order of 400 psi, the projections on the surface of the separator would stretch and rupture the membrane. The membrane would also lie tightly against the surface of the permeate separator, restricting the flow of the permeate. In the French design, only one active membrane surface is exposed to the flow of the feed solution. This severely limits the total amount of membrane area that can be placed in the spiral element. It would also limit the number of sheet assemblies or leaves that can be installed. An element that would contain a large surface area would also have an extremely long flow path which would create a relatively high pressure drop, and would limit the surface velocities of the feed water over the membrane surface.
Conventional commercial spiral wound reverse osmosis element designs and other spiral designs, such as disclosed in the Westmoreland Patent No. 3,367,504, use a grid or mesh in the feed flow path which obtructs or interferes with the free flow of feed water. The mesh or grid restricts the cross sectional area of the flow path. Turbulence is created between the strands. These effects combine to create high pressure drops in the feed channel at high velocities. The typical feed channel pressure drop in a 40 inch long reverse osmosis element is 10 psi at one foot/second and 220 psi at 4 feet/second. It is clear from this example that the surface velocities of the conventional spiral wound elements is limited to a maximum velocity of one foot per second. The pressure drop of the flow channel increases when the water contains suspended solids. This further limits the velocity of water in the feed channel. The water directly behind the strands of mesh is relatively stagnant. The suspended solids tend to deposit underneath the strands of the mesh. This fouls microfiltration, ultrafiltration, and reverse osmosis membranes with a thick layer of deposited solids buildup.